Switching control circuit and control method

ABSTRACT

A switching control circuit for controlling a multi-channel switching circuit can include: a logic control circuit that receives an external operation signal, and generates an enable signal, a trigger signal, and an order signal; a reference voltage regulation circuit that receives the enable signal, the trigger signal, the order signal, and a plurality of input voltage signals, and generates an adjustable reference voltage signal, where the reference voltage regulation circuit is also configured to select one of the plurality of input voltage signals based on the order signal; a feedback control circuit that receives the reference voltage signal, the plurality of input voltage signals, and the output voltage signal, and generates a feedback control signal; and a channel selection circuit that receives the order signal and the feedback control signal, and generates switching control signals to control switching operations of the multi-channel switching circuit.

RELATED APPLICATIONS

This application claims the benefit of Chinese Patent Application No.201610123766.8, filed on Mar. 8, 2016, which is incorporated herein byreference in its entirety.

FIELD OF THE INVENTION

The present invention generally relates to the field of switching powersupplies, and more particularly to switching control circuits andmethods.

BACKGROUND

Switch mode power supplies can efficiently convert electrical power froma source to a load, or to several different loads, with eachcorresponding to a different output. The main transistor of aswitching-mode supply can switch between on and off states at a givenoperating frequency, and voltage regulation can be achieved by varyingthe ratio of the on-to-off time of the main transistor. Switch modepower supplies may have relatively high power conversion efficiency, ascompared to other types of power converters. Switch mode power suppliesmay also be substantially smaller and lighter than a linear supply dueto the smaller transformer size and weight.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a schematic block diagram of a first example voltageswitching circuit.

FIG. 1B is a waveform diagram of example operation the circuit of FIG.1A.

FIG. 2A is a schematic block diagram of a second example voltageswitching circuit.

FIG. 2B is a waveform diagram of example operation of the circuit ofFIG. 2A.

FIG. 3A is a schematic block diagram of a third example voltageswitching circuit.

FIG. 3B is a waveform diagram of example operation of the circuit ofFIG. 3A.

FIG. 4 is a schematic block diagram of an example switching controlcircuit, in accordance with embodiments of the present invention.

FIG. 5 is a more detailed schematic block diagram of the exampleswitching control circuit, in accordance with embodiments of the presentinvention.

FIG. 6A is a schematic block diagram of an example reference voltageregulator configured for the switching control circuit of FIG. 5, inaccordance with embodiments of the present invention.

FIG. 6B is a waveform diagram of example operation of the circuit ofFIG. 6A, in accordance with embodiments of the present invention.

FIG. 7 is a waveform diagram of example operation of the circuit of FIG.5, in accordance with embodiments of the present invention.

DETAILED DESCRIPTION

Reference may now be made in detail to particular embodiments of theinvention, examples of which are illustrated in the accompanyingdrawings. While the invention may be described in conjunction with thepreferred embodiments, it may be understood that they are not intendedto limit the invention to these embodiments. On the contrary, theinvention is intended to cover alternatives, modifications andequivalents that may be included within the spirit and scope of theinvention as defined by the appended claims. Furthermore, in thefollowing detailed description of the present invention, numerousspecific details are set forth in order to provide a thoroughunderstanding of the present invention. However, it may be readilyapparent to one skilled in the art that the present invention may bepracticed without these specific details. In other instances, well-knownmethods, procedures, processes, components, structures, and circuitshave not been described in detail so as not to unnecessarily obscureaspects of the present invention.

In some applications with a plurality of input ports but only one outputport, different input power supplies may be switched in order to obtaina desired output voltage signal. Referring now to FIG. 1A, shown is aschematic block diagram of a first example voltage switching circuit.FIG. 1B shows a waveform diagram of example operation the circuit ofFIG. 1A. Here, the circuit with only two input ports is described as anexample, and the input ports may be switched based on the property ofunidirectional conductivity of diodes.

For each channel, one diode can be coupled to an input port at its anodeterminal, and to an output port at its cathode terminal. When voltageVIN1 at the first input port is required, voltage VIN1 can be coupled tothe input port, and voltage VOUT may be obtained by subtracting thevoltage drop of a diode from voltage VIN1. When voltage VIN2 at thesecond input port is required, voltage VIN2 can be coupled to the inputport, and if voltage VIN2 is greater than voltage VIN1, voltage VOUT canbe obtained by subtracting the voltage drop of a diode from voltageVIN2. Voltage VIN1 at the first input port may also be shut down orotherwise disabled in this case. In this approach, a diode voltage dropexists between voltage VOUT and voltage VIN1 (or VIN2) during operation,and relatively large power losses may be generated when a load currentis large. As a result, this approach may not be suitable forapplications having high accuracy demands or heavy load requirements.

Referring now to FIG. 2A, shown is a schematic block diagram of a secondexample voltage switching circuit. FIG. 2B, shows a waveform diagram ofexample operation of the circuit of FIG. 2A. For each channel, a MOStransistor may serve as a switching circuit that is coupled to an inputport. When voltage VIN1 at the first input terminal is required, the MOStransistor coupled to voltage VIN1 can be turned on, and when voltageVIN2 at the second input terminal is required, the MOS transistorcoupled to voltage VIN2 can be turned on while the MOS transistor in thefirst channel may be slightly turned off in advance. As compared to theabove approach, the system losses can be reduced by using thetransistors, but in order to achieve unidirectional conduction thatprevents current inversion, the MOS transistor may be turned off beforethe voltage is completely switched (e.g., time t1 to t3 in FIG. 2B).Thus, the output voltage may be less than the previous value by a diodevoltage drop (e.g., VOUT during time t1 to time t2). However, in someapplications (e.g., USB PD), even the voltage drop that equals a diodevoltage drop may still be far beyond the allowable range.

Referring now to FIG. 3A, shown is a schematic block diagram of a thirdexample voltage switching circuit. FIG. 3B shows a waveform diagram ofexample operation of the circuit of FIG. 3A. In this example, when theoutput voltage is switched to voltage VIN2 at the second input terminalfrom voltage VIN1 at the first input terminal, the transistor in thefirst channel may initially be turned off, and then the transistor inthe second channel can be turned on. Also, output voltage VOUT may bemaintained by an output capacitor during a predetermined time period(e.g., from time t1 to time t2 in FIG. 3B). However, during theswitching process there may be a drop in the output voltage, and thevalue of this drop can be related to the load, the output capacitor, andthe dead time of signals GATE1, GATE2. When the output capacitor has arelatively low value, unacceptable output voltage drop may be generated,and in some applications, the upper limit of the output capacitor may belimited, which can result in greater output voltage fluctuations.

In one embodiment, a switching control circuit for controlling amulti-channel switching circuit having input terminals respectivelycoupled to a plurality of input voltages, and output terminals coupledto an output terminal to provide an output voltage signal, can include:(i) a logic control circuit configured to receive an external operationsignal, and to generate an enable signal, a trigger signal, and an ordersignal; (ii) a reference voltage regulation circuit configured toreceive the enable signal, the trigger signal, the order signal, and aplurality of input voltage signals, and to generate an adjustablereference voltage signal, where the reference voltage regulation circuitis configured to select one of the plurality of input voltage signalsbased on the order signal; (iii) a feedback control circuit configuredto receive the reference voltage signal, the plurality of input voltagesignals, and the output voltage signal, and to generate a feedbackcontrol signal; and (iv) a channel selection circuit configured toreceive the order signal and the feedback control signal, and togenerate switching control signals to control switching operations ofthe multi-channel switching circuit.

Referring now to FIG. 4, shown is a schematic block diagram of anexample switching control circuit, in accordance with embodiments of thepresent invention. In this example, the switching control circuit isused to control a multi-channel switching circuit that includes firstthrough Nth switching circuits. The first switching circuit can includetransistors Q1 and Q2, the second switching circuit can includetransistors Q3 and Q4, and so on through the Nth switching circuit,which can include transistors QM and QN. The multi-channel switchingcircuit includes input terminals respectively coupled to a plurality ofinput voltages (e.g., VIN_1 through VIN_N). Also, output terminals ofall channels of the multi-channel switching circuit can be coupled to acommon node as an output terminal to provide expected output voltagesignal VOUT.

The switching control circuit can include logic control circuit 41,reference voltage regulator 42, feedback control circuit 43, and channelselection circuit 44. For example, logic control circuit 41 can receiveexternal operation signal “Order,” and may generate enable signal EN,trigger signal EN_tra, and order signal CS_Order. External operationsignal Order can represent operation information (e.g., start, channelswitch, etc.). When the external operation signal is asserted, enablesignal EN can be activated, the trigger signal can be activated ordeactivated based on the external operation signal, and the order signalcan output the channel switching information according to the externaloperation signal. For example, logic control circuit 41 may include atrigger, a delay circuit, and/or components with similar functionality.

Reference voltage regulator 42 can receive enable signal EN, triggersignal EN_tra, order signal CS_Order, and a plurality of input voltagesignals VIN_1 to VIN_N, and may generate adjustable reference voltagesignal SST_ref. Reference voltage regulator 42 can determine which oneof a plurality of input voltage signals should be coupled in accordingto order signal CS_Order. Also, the reference voltage signal may bedifferent when the input voltage signals are different. Feedback controlcircuit 43 can receive reference voltage signal SST_ref, a plurality ofinput voltage signals VIN_1 to VIN_N, and output voltage signal VOUT ofthe multi-channel switching circuit, and may generate feedback controlsignals OP1_OUT and OP2_OUT. Channel selection circuit 44 can receiveorder signal CS_Order and feedback control signals OP1_OUT and OP2_OUT,and may generate switching control signals in order to control theswitches of the multi-channel switching circuit. For example, thechannel selection circuit can be a multiplexer circuit.

Referring now to FIG. 5, shown is a more detailed schematic blockdiagram of the example switching control circuit, in accordance withembodiments of the present invention. In this example, feedback controlcircuit 43 can include a first feedback circuit having a firstoperational amplifier and a second feedback circuit having a secondoperational amplifier. The first operational amplifier may have anon-inverting input terminal for receiving reference voltage signalSST_ref, and an inverting input terminal for receiving output voltagesignal VOUT of the multi-channel switching circuit, so as to outputfeedback control signal OP1_OUT. The second operational amplifier mayhave a non-inverting input terminal for receiving a minimum value of aplurality of input voltage signals, such as ON_min (VIN_1 . . . VIN_N),and an inverting input terminal for receiving the output voltage signal,so as to output feedback control signal OP2_OUT. The minimum valueherein can be the smaller one between two input voltage signals to beswitched. In this particular example, the feedback control circuit mayalso include voltage source Vdrop having a cathode coupled to an outputterminal of the multi-channel switching circuit, and an anode coupled toan inverting input terminal of the second operational amplifier.

Referring now to FIG. 6A, shown is a schematic block diagram of anexample reference voltage regulator configured for the switching controlcircuit of FIG. 5, in accordance with embodiments of the presentinvention. In this example, the reference voltage regulator can includelogic controller 61, multiple-channel selection circuit 62, and a chargeand discharge circuit. Logic controller 61 may receive enable signal EN,trigger signal EN_tra, order signal CS_Order, and feedback signalOP2_OUT, and may generate charge signal CT_CHG and discharge signalCT_DIS. The logic controller may be a circuit implemented by logiccomponents (e.g., a trigger, a delay circuit, etc.). Themultiple-channel selection circuit can determine which one of aplurality of input voltage signals is to be coupled in according toorder signal CS_Order.

The charge and discharge circuit can include charge current source Ia,discharge current source Ib, switch Q11, and capacitor CT. chargecurrent source Ia may have a first terminal coupled to an outputterminal of the multiple-channel selection circuit, and a secondterminal coupled to a first terminal of the first capacitor. Dischargecurrent source Ib, switch Q11, and capacitor CT can be coupled inparallel, and the second terminal of capacitor CT can connect ground.Also, charge current source Ia can receive charge signal CT_CHG, and maycharge capacitor CT when the charge signal is active. Discharge currentsource Ib can discharge signal CT_DIS, and may discharge capacitor CTwhen the discharge signal is active.

Referring now to FIG. 6B, shown is a waveform diagram of exampleoperation of the circuit of FIG. 6A, in accordance with embodiments ofthe present invention. In this example of two channels, the firstswitching circuit can initially be operational, and then this isswitched to the second switching circuit. Also, input voltage VIN1 isless than input voltage VIN2 in this particular example. At time t0,order signal CS_Order can indicate the first switching circuit isenabled (e.g., channel CH1_ON is active). Input voltage VIN1 may beprovided to multiple-channel selection circuit 62, and be transferred asthe output voltage to charge current source Ia via the multiple-channelselection circuit. Enable signal EN can be active high, charge signalCT_CHG output by the logic controller can also be active high, andcharge current source Ia may charge capacitor CT. Thus, the voltage atthe first terminal of capacitor CT may linearly increase to VIN1, andthe voltage across capacitor CT may be provided to feedback controlcircuit 43 as reference voltage signal SST_ref.

At time t1, order signal CS_Order can indicate that the second switchingcircuit is enabled (e.g., channel CH2_ON is active). Input voltage VIN2may be provided to multiple-channel selection circuit 62, and can becoupled to charge current source Ia. Enable signal EN may remain high,and trigger signal EN-tra can go active high. The charge signal turnscan be deactivated to a low level, and the discharge signal may beactivated to a high level. Charge current source Ia may be disabled, anddischarge current source Ib can discharge capacitor CT, such that thevoltage across capacitor CT (e.g., reference voltage signal SST_ref) canbegin decreasing until time t2. Feedback control signal OP2_OUT can goactive high, and logic controller 61 may activate switching controlsignal VQ to turn on switch Q11, and reference voltage signal SST_refmay rapidly decrease to zero. At this time, charge signal CT_CHG can gohigh, discharge signal CT_DIS can go low, and charge current source Iamay charge capacitor CT. Also, the voltage at the first terminal ofcapacitor CT may linearly increase to VIN2, and the voltage acrosscapacitor CT can be provided to feedback control circuit 43 as referencevoltage signal SST_ref.

Thus, the regulation of the reference voltage signal may be completedduring the switch from the first switching circuit to the secondswitching circuit, and the variation procedure of the signals during theswitching from the second switching circuit to the first switchingcircuit may be as shown from time t3 to time t5. In this example, as thereference voltage signal is regulated along with the change of the inputvoltage, the associated change of the output voltage can be performedduring the switching of the input voltage so as to reduce the conversiontime.

The following may describe the specific switching procedure betweendifferent input voltages in conjunction with FIGS. 5, 6B, and 7according to embodiments of the present invention. In this example, thecircuit with two channels is described, and the system is started withthe first switching circuit enabled, then the operation is switched fromthe first switching circuit to the second switching circuit, followed bybeing switched from the second switching circuit back to the firstswitching circuit. Further, the first input voltage is less than thesecond input voltage in this particular example.

In one embodiment, a method of controlling a multi-channel switchingcircuit having input terminals respectively coupled to a plurality ofinput voltages, and output terminals coupled to an output terminal toprovide an output voltage signal, can include: (i) receiving anoperation signal, and generating an enable signal, a trigger signal, andan order signal; (ii) generating an adjustable reference voltage signalbased on the enable signal, the trigger signal, the order signal, andthe plurality of input voltage signals; (iii) selecting one of the inputvoltage signals according to the order signal; (iv) generating afeedback control signal in response to the reference voltage signal, theplurality of input voltage signals, and the output voltage signal; and(v) generating switching control signals to control the switches of themulti-channel switching circuit in response to the order signal and thefeedback control signal.

Referring now to FIG. 7, shown is a waveform diagram of exampleoperation of the circuit of FIG. 5, in accordance with embodiments ofthe present invention. At time t0, logic control circuit 41 can receiveoperation signal Order, and order signal CS_Order may indicate the firstchannel CH1_ON. Channel selection circuit 44 may receive order signalCS_Order, and can transmit feedback control signal OP1_OUT to driver 1,and enable signal EN can go active high. During the operation procedureof the reference voltage regulation circuit, reference voltage signalSST_ref can begin rising, and then feedback control signal OP1_OUT, theoutput of driver 1, and output voltage VOUT, can begin rising. At timet1, reference voltage signal SST_ref can increase to the level of VIN1,feedback control signal OP1_OUT can reach its maximum value, switches Q1and Q2 of the first switching circuit may be fully on, and the value ofoutput voltage VOUT can be VIN1.

At time t2, operation signal “Order” may indicate the second switchingcircuit is enabled, order signal CS_Order can indicate channel CH2_ON,and trigger signal EN_tra may go active high. Also, discharge signal DIScan be activated at a high level, the reference voltage signal SST_refcan begin to decrease, and feedback control signal OP1_OUT may decrease.Discharge circuit 45 may be enabled, discharge switch Qdis can turn on,and the discharge resistor can discharge output voltage VOUT to decreasethe level of VOUT. In order to facilitate the control the dischargecircuit, the discharge time of discharge circuit 45 may be set (e.g., bya user based on circuit requirements) to be a predetermined value.

At time t3, output voltage VOUT can decrease to the difference betweenVIN1 and Vdrop, and feedback control signal OP2_OUT can go active high.Channel selection circuit 44 can couple feedback control signal OP2_OUTto driver 1, and feedback control signal OP1_OUT to driver 2, andreference voltage signal SST_ref may be reduced to zero. From time t3 tot4, reference voltage signal SST_ref may gradually rise, and the outputof driver 2 can also gradually rise in the function of feedback controlsignal OP1_OUT. In this way switches Q3 and Q4 of the second switchingcircuit can be “softly” or slowly started/enabled. In addition, outputvoltage VOUT may remain at the value of VIN1−Vdrop at this time.

Because the smaller one of VIN1 and VIN2 (e.g., VIN1) is input to thepositive input terminal of the second operational amplifier, and voltageOUT+Vdrop is input to the inverting input terminal, output voltage VOUTmay remain at VIN1−Vdrop based on the operating principles of theoperational amplifier. Voltage Vdrop of the first voltage source can bea drop voltage of output voltage OUT set by the system, and the valuemay be far less than the diode voltage drop. In this way, relativelylarge voltage drops of output voltage VOUT can be substantially avoidedduring the switching procedure.

At time t4, reference voltage signal SST_ref may rise to a certain value(e.g., VIN1−Vdrop), and the output of driver 2 can increase. Also,switches Q3 and Q4 of the second switching circuit may be graduallyturned on, and finally, output voltage VOUT may be greater than thedifference between VIN1 and Vdrop. When output voltage VOUT reaches thedifference between VIN1 and Vdrop, the second feedback control signalOP2_OUT may go inactive low, the output of driver 1 may be graduallydecreased to zero, and the switches of the first switching circuit maybe off. In this configuration, switches of the first switching circuitcan be completely off before the output voltage rises to input voltageVIN1 because of voltage Vdrop. Thus, the time period during which theoutput of driver 1 is decreased to zero is less than the time periodduring which output voltage VOUT is increased to VIN1 by driver 2. As aresult, the occurrence of the output voltage being greater than thefirst input voltage even the switches of the first switching circuit arenot off can be substantially avoided, so as to prevent the current fromflowing back to the first input terminal from output OUT.

From time t4 to time t5, reference voltage signal SST_ref may continuerising to VIN2, and feedback voltage OP1_OUT can continue rising untilswitches Q3 and Q4 are completely turned on by driver 2. Output voltageVOUT can reach input voltage VIN2, and thus the switching operation fromthe first switching circuit to the second switching circuit may becomplete. At time t6, operation signal “Order” can indicate that thefirst switching circuit is to be enabled, order signal CS_Order canindicate channel CH1_ON, and trigger signal EN_tra may go active high.Based on a similar procedure, reference voltage signal SST_ref can beginto decrease by discharging, and feedback control signal OP1_OUT canaccordingly decrease. Discharge circuit 45 may be enabled, dischargeswitch Qdis can turn on, and the discharge resistor can discharge outputvoltage VOUT such that output voltage VOUT decreases.

At time t7, output voltage VOUT can be decreased to a difference betweenVIN1 and Vdrop, feedback control signal OP2_OUT may go active high, andthe channel selection circuit 44 can couple feedback control signalOP1_OUT to driver 1, and feedback control signal OP2_OUT to driver 2.Also, reference voltage signal SST_ref can be decreased to zero. Fromtime t7 to t8, reference voltage signal SST_ref may gradually rise, andthe output of driver 1 can also gradually rise in the function offeedback control signal OP1_OUT, such that switches Q1 and Q2 of thefirst switching circuit can be started softly. At the same time, outputvoltage OUT may remain at the value of VIN1−Vdrop.

At time t8, reference voltage signal SST_ref can rise to a certain value(e.g., VIN1−Vdrop), and the output of driver 1 can increase. Switches Q1and Q2 of the first switching circuit may be gradually turned on, andfinally, output voltage VOUT may be greater than the difference betweenVIN1 and Vdrop. When output voltage VOUT reaches the difference betweenVIN1 and Vdrop, feedback control signal OP2_OUT may be deactivated, theoutput of the driver 2 can be gradually decreased to zero, and theswitches of the second switching circuit may be off. From time t8 to t9,output voltage VOUT may gradually rise by the control of feedbackcontrol signal OP1_OUT, and reach the level of VIN1 at time t9, wherebyswitches Q1 and Q2 of the first switching circuit are fully on. In thisway, the system can be switched from the second switching circuit to thefirst switching circuit, and the output voltage may remain almost stablewithout any substantial voltage drop during the switching procedure.

At time t10, the order signal Order may indicate that the system is tobe off, the reference voltage signal SST_ref may begin to linearlydecrease. The discharge circuit can be enabled, and the dischargeresistor can discharge output voltage VOUT until output voltage VOUT isdecreased to zero. Channel selection circuit 44 may disconnect feedbackcontrol signal OP2_OUT from driver 2, and also disconnect feedbackcontrol signal OP1_OUT from driver 1. At time t12, the system can bedisabled.

According to the above switching control circuits and methods, whendifferent input power sources are to be switched, the logic controlcircuit may output the order signal for switching, the reference voltagesignal can receive the order signal, the trigger signal, and the secondfeedback control signal, in order to regulate the reference voltagesignal based on the different input voltages. In this way, the outputvoltage can be controlled to change slowly and steadily until reaching alevel of the expected output voltage, in order to substantiallyguarantee that the output voltage does not drop and remains stableduring the switching between different channels.

The embodiments were chosen and described in order to best explain theprinciples of the invention and its practical applications, to therebyenable others skilled in the art to best utilize the invention andvarious embodiments with modifications as are suited to particularuse(s) contemplated. It is intended that the scope of the invention bedefined by the claims appended hereto and their equivalents.

What is claimed is:
 1. A switching control circuit for controlling amulti-channel switching circuit having input terminals respectivelycoupled to a plurality of input voltages, and output terminals coupledto an output terminal to provide an output voltage signal, the switchingcontrol circuit comprising: a) a logic control circuit configured toreceive an external operation signal, and to generate an enable signal,a trigger signal, and an order signal; b) a reference voltage regulationcircuit configured to receive said enable signal, said trigger signal,said order signal, and a plurality of input voltage signals, and togenerate an adjustable reference voltage signal, wherein said referencevoltage regulation circuit is configured to select one of said pluralityof input voltage signals based on said order signal; c) a feedbackcontrol circuit configured to receive said reference voltage signal,said plurality of input voltage signals, and said output voltage signal,and to generate a feedback control signal; and d) a channel selectioncircuit configured to receive said order signal and said feedbackcontrol signal, and to generate switching control signals to controlswitching operations of said multi-channel switching circuit.
 2. Theswitching control circuit of claim 1, wherein said feedback controlcircuit comprises: a) a first feedback circuit comprising a firstoperational amplifier configured to receive said output voltage signaland said reference voltage signal, and to generate a first feedbackcontrol signal; and b) a second feedback circuit comprises a secondoperational amplifier configured to receive said output voltage signaland a minimum value of said input voltage signals, and to generate asecond feedback control signal, wherein said minimum value of said inputvoltage signals is the minimum one of two input voltage signals to beswitched.
 3. The switching control circuit of claim 2, wherein saidadjustable reference voltage signal is generated by a reference voltageregulation circuit comprising: a) a logic controller configured toreceive said enable signal, said trigger signal, said order signal, andsaid second feedback control signal, and to generate a charge signal anda discharge signal; b) a multiple-channel selection circuit configuredto select one of said input voltage signals according to said ordersignal; c) a charge and discharge circuit comprising a charge currentsource, a discharge current source, a first switch, and a firstcapacitor, wherein said charge current source has a first terminalcoupled to said output terminal, and a second terminal coupled to afirst terminal of said first capacitor; d) said discharge current sourcebeing coupled in parallel with said first switch and said firstcapacitor, and wherein a second terminal of said first capacitor isgrounded; e) said charge current source being configured to receive saidcharge signal, and to charge said first capacitor when said chargesignal is active; and f) said discharge current source being configuredto receive said discharge signal, and to discharge said first capacitorwhen said discharge signal is active.
 4. The switching control circuitof claim 2, wherein said second feedback circuit is configured toreceive said trigger signal, and to be disabled when said trigger signalis inactive.
 5. The switching control circuit of claim 1, wherein saidchannel selection circuit comprises a multiplexer.
 6. The switchingcontrol circuit of claim 1, wherein said switching control circuitfurther comprises a discharge circuit having a discharge resistor and adischarge switch that are coupled in series between said output terminaland ground, wherein said discharge switch is controlled by a dischargecontrol signal generated by said logic control circuit.
 7. The switchingcontrol circuit of claim 2, wherein said feedback control circuitfurther comprises a first voltage source having a cathode coupled tosaid output terminal, and an anode coupled to an inverting inputterminal of said first operational amplifier.
 8. A method of controllinga multi-channel switching circuit having input terminals respectivelycoupled to a plurality of input voltages, and output terminals coupledto an output terminal to provide an output voltage signal, the methodcomprising: a) receiving an operation signal, and generating an enablesignal, a trigger signal, and an order signal; b) generating anadjustable reference voltage signal based on said enable signal, saidtrigger signal, said order signal, and said plurality of input voltagesignals; c) selecting one of said input voltage signals according tosaid order signal; d) generating a feedback control signal in responseto said reference voltage signal, said plurality of input voltagesignals, and said output voltage signal; and e) generating switchingcontrol signals to control the switches of said multi-channel switchingcircuit in response to said order signal and said feedback controlsignal.
 9. The method of claim 8, wherein said generating said feedbackcontrol signal comprises: a) generating, by a first operationalamplifier, a first feedback control signal, wherein said firstoperational amplifier receives said output voltage signal and saidreference voltage signal; and b) generating, by a second operationalamplifier, a second feedback control signal, wherein said secondoperational amplifier receives said output voltage signal and a minimumvalue of said input voltage signals, wherein said minimum value of saidinput voltage signals is the minimum one of two input voltage signals tobe switched currently.
 10. The method of claim 9, wherein saidgenerating said adjustable reference voltage signal comprises: a)generating a charge signal and a discharge signal in response to saidenable signal, said trigger signal, said order signal, and said secondfeedback control signal; b) coupling said selected input voltage signalto a charge current source; c) charging, by said charge current source,a first capacitor when said charge signal is active; and d) discharging,by a discharge current source said first capacitor when said dischargesignal is active, wherein a voltage across said first capacitor isconfigured as said reference voltage signal.